The present application claims priority to Japanese Application No. P11-120074 filed Apr. 27,1999, which application is incorporated herein by reference to the extent permitted by law.
1. Field of the Invention
The present invention relates to a semiconductor device and method of production of the same, more particularly relates to an improvement or increasing the strength of a bond of a metal bump.
2. Description of the Related Art
In chip size packages (CSP) and other semiconductor devices of the flip-chip type, wiring surfaces of the IC chips directly face the boards and are bonded conductively with wiring patterns of the boards via conductor bonds formed by solder, gold, copper, or alloys of the same. However, a semiconductor IC chip is generally comprised by silicon and therefore has a linear expansion rate with respect to heat much different from that of a circuit board, which is comprised mainly of a resin, so stress arises due to the difference of the amount of expansion by the heat received from the chip and surroundings at the time of actual use. In most cases, the portions most susceptible to stress are the bonds between the packaged IC chip and the board. In particular, in the case of the flip-chip type, since there are no leads and few portions for relieving stress, it is necessary to improve the reliability of the bonds.
In the flip-chip type package of the related art, the reliability of the bonds was increased by the method of filling the space between the board and the chip with a resin so as to increase the reliability of the bonds. However, this method made the reworkability of the package worse. Accordingly, a method of bonding a board and an IC chip by only solder which gives a high reliability of bonds has been desired.
Among flip-chip types, in a structure such as an encapsulated bump chip (EBC) to be packaged at the wafer level (see later explained FIG. 6A), in the method of production, a resin is applied for protecting the wafer surface by spin-coating, so the thickness of the coating resin is affected by the Young""s modulus and viscosity of the resin.
The material properties of the resin may be selected in innumerable ways. If just using the resin as a wiring protection film, a resin having a low Young""s modulus and low viscosity may be used, but to give the resin the function of improving the bond reliability of the bumps and impart the property of enabling the resin to be coated to a uniform height within a predetermined time at the time of the spin-coating, it is necessary to suitably select the resin used.
Further, the thickness of coating of the resin has a large effect on the action of reinforcing the bonding surface of the bumps, particularly the bonding surface with the IC chip, due to the later explained reason, so it is necessary to set a suitable thickness of coating.
FIG. 6A shows the state of bonding between the solder bumps of the semiconductor package (EBC) and board.
A semiconductor chip 2 is mounted on the board 1. The semiconductor chip 2 is bonded on the board 1 via primary solder bumps 3 and secondary solder bumps 4.
When the semiconductor package (semiconductor device) drops in temperature, a contraction force acts on the board 1 and the semiconductor chip 2 as shown by A and B, respectively. In this case, due to the difference of the linear expansion rates, the amount of contraction A of the board 1 becomes larger than the amount of contraction B of the semiconductor chip 2. Accordingly, as shown in FIG. 6B, due to the difference of expansion (difference of amounts of contraction), the package deforms and a tensile stress C arises at the bonds of the solder bumps as shown in FIG. 6C.
There are three locations which may be destroyed by the stress received by the solder bumps: between the chip and the solder, between the board and the solder, and between the primary bumps and the secondary bumps. In forming the bumps, however, if the bonding areas are equal, the location which may be destroyed first usually becomes the location between the chip and the solder. The reason is that on the board side, the Young""s modulus of the board itself is low, so the strain is absorbed immediately below the solder bonding surface, and, on the IC chip bonding surface side, there is no relief mechanism for absorbing such strain. Further, if the solder and the for example Cu of the bonds are exposed to a high temperature to form an alloy, the layer ends up becoming brittle, and, in the production process, the frequency of exposure to heat is greater at the bonding surface with the IC.
Namely, in a structure of the flip-chip type, when maintaining the bond with the board by solder bonding, if the bonding areas between the IC chip and the solder and between the board and the solder are substantially the same, it is important to increase the bond strength between the IC chip and the solder in order to improve the solder bonds life.
The state of occurrence of stress between the IC chip and solder is shown in FIGS. 6B and 6C as explained above.
For example, assume that a board mounted with an IC chip is cooled from 100xc2x0 C. to 0xc2x0 C. The deformation of the board at this time becomes the warping state as shown in FIG. 6B due to the larger amount of contraction of the board compared with the IC chip. The stress mainly acting on the solder in this case is the tensile stress as shown in FIG. 6C.
Summarizing the problems to be solved by the invention, as a result of such a tensile stress, the base portion of the solder bond easily cracks making destruction of that portion easier. In particular, when a bump is shaped so that base portion of the solder narrows and therefore becomes thinner than the center portion, the effect of such heat stress becomes larger.
An object of the present invention is to provide a semiconductor device comprising a chip bonded to a board via bumps wherein the reliability of the bump bonds is improved and a method of production of the same.
According to a first aspect of the present invention, there is provided a semiconductor device comprising a semiconductor substrate on which metal bumps having narrowed base portions are bonded and coated on its top surface with a resin covering the base portions of the metal bumps.
Preferably, the resin has a Young""s modulus of not less than about 1200 kgf/mm2, a linear expansion rate of not more than about 20 ppm/xc2x0 C., and a thickness of about 40 xcexcm to about 110 xcexcm.
Preferably, the metal bumps have polished upper surfaces and a difference in height between the polished upper surfaces and a surface at the smallest coating thickness portion of the resin is about 10 xcexcm to about 50 xcexcm.
Preferably, the resin is inclined in a flared shape at the base portions of the metal bumps and buries the base portions.
According to a second aspect of the present invention, there is provided a method of production of a semiconductor device including the steps of forming metal bumps in a wafer state, coating a resin for protecting a wiring surface of a semiconductor chip in the wafer state and cutting the semiconductor chip from the wafer, wherein the metal bumps have narrowed base portions and are bonded to the wiring surface of the semiconductor chip and the resin is coated covering the base portions of the metal bumps.
Preferably, the resin is coated by a spin coating method, and a Young""s modulus of the resin is not less than about 1200 kgf/mm2, a linear expansion rate is not more than about 20 ppm/xc2x0 C., and a thickness is about 40 xcexcm to about 110 xcexcm.
Preferably, the method further comprises a step of polishing surfaces of the metal bumps in the water state, wherein a difference in height between the polished surfaces and a surface at the smallest coating thickness portion of the resin is about 10 xcexcm to about 50 xcexcm.
Preferably, the resin is coated so that the resin is inclined in a flared shape at the base portions of the metal bumps and buries the base portions.